Method and apparatus for pre-heating lng boil-off gas to ambient temperature prior to compression in a reliquefaction system

ABSTRACT

A method and apparatus of pre-heating LNG boil-off gas stream flowing from a reservoir in a reliquefaction system, before compression. The method comprises heat exchanging the BOG stream in a first heat exchanger, against a second coolant stream having a higher temperature than the BOG stream, where the second coolant stream is obtained by selectively splitting a first coolant stream into second and third coolant streams, third coolant stream being flowed into a first coolant passage in a reliquefaction system cold box, whereby the BOG has reached near-ambient temperatures prior to compression and the low temperature duty from the BOG is substantially preserved within the reliquefaction system, and thermal stresses in the cold box are reduced. Before the compression step, the BOG is pre-heated to substantially ambient temperatures, by heat exchanging the BOG with said coolant, said coolant prior to the heat exchange having a higher temperature than the BOG.

FIELD OF THE INVENTION

The invention relates to the field of re-liquefaction of boil-off gases from liquid natural gas (LNG). More specifically, the invention relates to a method and an apparatus for pre-heating LNG boil-off gas (BOG) stream flowing from a reservoir in a reliquefaction system, prior to compression, and a method and an apparatus for cooling an LNG boil-off gas (BOG) stream in a reliquefaction plant.

Background

A new generation of LNG vessels was established in association with the introduction of LNG reliquefaction systems (LNG RS). Prior to this, basically all LNG vessels were driven by steam turbines fuelled by boil off gases (BOG) evaporating from the cargo during transportation. In periods when the total amount of BOG was insufficient to cover the entire power demand, additional LNG had to be fed to the boilers through forced vaporizers.

Brief Description of the Prior Art

The new LNG RS opened the possibility to collect, cool down and reliquefy all BOG and hence preserve the total cargo volume throughout the laden and ballast voyages. Conventional slow speed diesel engines, with high efficiencies compared to the steam turbines, could then be used for propulsion.

Several patents have described various aspects with such reliquefaction plants, and accordingly improvements to these. The prior art (e.g. Norwegian Patent Application No. 20051315 basically focuses on improvements of the nitrogen Brayton cycle and the utilization of cold nitrogen for pre-cooling. There is, however, a further need to improve the system in order to reduce the power demands.

Most of today's LNG vessels utilize low-temperature centrifugal BOG compressors to feed their boilers. Much of the reason for choosing low-temperature compression is that this will reduce the compressor size significantly compared to compression at ambient temperatures. The fan laws are applicable for centrifugal compressors, and show that a low suction temperature will ensure a higher pressure ratio per stage. The density of the gas will accordingly increase, the volume flow is reduced to a minimum, and the size and efficiency of the BOG compressors become more favourable.

Since there is no need to preserve the low temperature duty in the BOG stream—in fact the BOG is normally additionally heated before introduction to the boilers—the heat of compression is deliberately absorbed by the compressed gas without any means of heat rejection downstream the BOG compression.

The common practice of low.temperature BOG compression has been further applied to new BOG compressor designs, dedicated for operation towards LNG reliquefaction systems. From an energy point-of-view this results in inefficient operation, since the cooling cycle must be sized to remove the heat of compression from BOG compressors, in addition to the heat of evaporation and the superheating adsorbed in the cargo containment system.

Also, other problems arise when low-temperature BOG compression is applied. Since no aftercoolers (intercoolers) are employed, recycling at low capacities depend on temperature control upstream the BOG compressor. The cooling duty necessary for this purpose can be difficult to predict since it will depend much on the BOG compressor efficiency, which in turn depends on several properties of the processed stream. Using recondensed BOG to provide this cooling, also reduces the performance of the plant, measured in terms of power per unit reliquefied BOG returned to the tanks.

SUMMARY OF THE INVENTION

It is thus provided a method of A method of pre-heating LNG boil-off gas (BOG) stream flowing from a reservoir in a reliquefaction system, prior to compression, the method comprising heat exchanging the BOG stream in a first heat exchanger, against a second coolant stream having a higher temperature than the BOG stream, the method being characterized in that the second coolant stream is obtained by selectively splitting a first coolant stream into said second coolant stream and a third coolant stream, said third coolant stream being flowed into a first coolant passage in a reliquefaction system cold box, whereby the BOG has reached near-ambient temperatures prior to compression and heat exchange with low temperature BOG is done by optimising the split of the coolant in the first heat exchanger in order to minimize exergy losses, and thermal stresses in the cold box are reduced.

It is also provided a method for cooling an LNG boil-off gas (BOG) stream in a reliquefaction plant, the BOG flowing from a reservoir, the method comprising compressing the BOG; heat exchanging the compressed BOG against a coolant in a cold box; flowing substantially re-liquefied BOG from the cold box to the reservoir, characterized by prior to the compression step, pre-heating the BOG to substantially ambient temperatures, by heat exchanging the BOG with said coolant, said coolant prior to the heat exchange having a higher temperature than the BOG.

In one embodiment, the pressure of the reliquefied BOG between the cold box and the reservoir is controlled independently of the BOG compressor discharge pressure and the reservoir pressure, and the amount of vent gas generated and the vent gas composition thus may be controlled.

It is also provided an apparatus for cooling an LNG boil-off gas (BOG) in a reliquefaction system, comprising a closed-loop coolant circuit for heat exchange between a coolant and the BOG; a BOG compressor having an inlet side fluidly connected to an LNG reservoir; a cold box having a BOG flowpath with a BOG inlet fluidly connected to the BOG compressor outlet side; said BOG flowpath having outlet for substantially re-liquefied BOG, fluidly connected to the reservoir; said cold box further comprising coolant flowpaths for heat exchange between the BOG and the coolant; characterized by a first heat exchanger in the fluid connection between the reservoir and the BOG compressor inlet side, said first heat exchanger having a coolant path fluidly connected to the closed-loop coolant circuit, at a point downstream of the coolant circuit's compander aftercooler but upstream of the coolant flow paths in the cold box, whereby the BOG compressor receives BOG with temperatures near or at the system ambient temperatures.

In one embodiment, the invention provides a separator in fluid connection with the cold box outlet and with the reservoir, a first valve in the cold box outlet line and a second valve in a line connected to the reservoir, said separator also comprising a vent line (11), whereby the pressure in the separator may be controlled, and the amount of vent gas and the vent gas composition thus may be adjusted.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified process flow diagram, illustrating the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will now be described with reference to FIG. 1, illustrating the novel features of the LNG RS with ambient temperature BOG compression. The figure shows schematic a cargo tank 74, holding a volume of LNG 72. BOG, evaporating from the LNG, enters a line 1 which is connected to a first heat exchanger H10. In this heat exchanger, the BOG is heated up to near-ambient temperatures, as will be described later. Following this pre-heating, the BOG enters the first stage BOG compressor C11 via line 2. The BOG compressor is shown as a three-stage centrifugal compressor C11, C12, C13, interconnected via lines 3-7 via intercoolers H11, H12 and aftercooler H13 as shown in the figure, but other compressor types may be equally applicable. The pre-heating ensures that the heat generated by the compression may be rejected through cooling water in the intercoolers H11, H12 and the aftercooler H13.

Pressurized BOG is then, via a line 8, fed into a second heat exchanger (or “cold box”) H20 where it is heat exchanged against a coolant, as will be described later. The coolant is preferably nitrogen (N₂). Following heat exchange, substantially reliquefied BOG exits the cold box H20 via a lines 9, 10 connected to a separator F10. The separator is provided with a vent line 11. A throttling valve V10 is arranged in the lines 9, 10 between the cold box and the separator, for expanding the reliquefied BOG. Following separation, reliquefied BOG is fed into the LNG 72 in the cargo tank 74 via lines 12, 13, as shown in FIG. 1. A valve V11 is arranged in the lines between the separator F10 and the tank 74, the purpose of which will be described later.

The closed N₂-Brayton cooling cycle is here represented by a 3-stage compressor C21, C22, C23 with intercoolers H21, H22, aftercooler H23, interconnected via lines 51-55 as shown in the figure, and a single expander stage E20. (Other cooling cycle constellations, for instance as discussed in Norwegian Patent Application No. 20051315 can also be utilized in this context.) Pressurized coolant (N₂) exits the compressor and the aftercooler H23 via a line 56 connected to a three-way valve V12. The three-way valve V12 is controllable to selectively split the high-pressure N₂ stream flowing in the line 56 into two different streams in respective lines 57, 59, as further detailed below. A first outlet of the three-way valve V12 is connected to a coolant inlet in the first heat exchanger H10 via a line 59. A line 60 connects the coolant outlet of the first heat exchanger H10 with the second heat exchanger's H20 middle section, via line 61, as shown in FIG. 1. A line 57 connects a second outlet of the three-way valve V12 to the inlet of a first coolant passage 82 in the second heat exchanger H20 upper section. The first coolant passage 82 outlet is connected via a line 58 to an entry point on the line 60 described above. A line 61 connects this entry point to a the inlet of a second coolant passage 84 in the cold box, in the vicinity of the cold box' middle section, as illustrated by FIG. 1. Coolant flows through the second coolant passage 84 and into an expander E20 via a line 62. The expanded coolant enters the second heat exchanger (cold box) H20 lower section via a line 63 connected to the inlet of a third coolant passage 86 before it exits the heat exchanger and flows back to the compressor C21, C22, C23 via the line 50. The flow split here described as a three-way valve V12 can equally be performed by other flow control configurations, such as normal single line control valves, orifices, etc. The important aspect is that the flow split can be controlled in order to cope with varying BOG flow conditions.

Generally, the process involves three new features which differ from previously suggested reliquefaction designs:

-   1. A heat exchanger H10, to ensure that most of the low-temperature     duty which can be extracted from the BOG in the ship's vapor header     line 1, remains preserved within the reliquefaction system, -   2. A BOG compressor C11, C12, C13 working under ambient, or     near-ambient conditions, with rejection of its heat of compression     H11, H12, H13 to the ambience; -   3. A generally higher pressure for the BOG stream 8 entering the     main heat exchanger (cold box) H20, compared to the discharge     pressure of common BOG compressors, allowing the condensation to     take place at a higher temperature level, and at the same time opens     the possibilities for controlling the separation pressure in the     separator F10 at a level between the cold box outlet pressure in the     line 9 and the storage pressure in the cargo tanks 74. This pressure     control must be seen in association with flow control through the     separator vent line 11 (flow control valve not shown in FIG. 1). By     adjusting the separation pressure, the vent flow, as well as the     composition of the condensate which is returned to tanks 74, can be     controlled according to the operator preferences. Minimizing the     vent gas flow results in higher required reliquefaction power input     and vice versa. Adjustments of the separator pressure will therefore     allow the operator to select the most favourable conditions for     economic optimization of the LNG RS operation.

1. Heat Exchanger Upstream BOG Compressor

The heat exchanger H10 upstream the BOG compressor C11, C12, C13 is installed to preserve the low-temperature duty in the BOG coming from the tanks 74, within the system. To extract as much low temperature duty as possible from this BOG stream, the BOG temperature should be allowed to increase up to near-ambient temperatures. To preserve the low temperature duty within the system, the duty must be absorbed by another stream in the reliquefaction system, originating at a higher temperature than the BOG stream.

This other stream will typically be a fraction of the warm high-pressure N₂-stream 59 as shown in FIG. 1. Other alternatives, such as using the entire N₂-stream (not only a part of it), or the BOG-stream from downstream the BOG compressor's aftercooler are also possible. However, the process of FIG. 1 will probably be the most beneficial, given the limitations and characteristics of commonly employed equipment for such processes. Consequently, only the process of FIG. 1, involving a split of the high-pressure N₂-stream 56 downstream the N₂-compander's aftercooler H23 into two different streams 57, 59, will be discussed next.

The BOG pre-heater control is based on controlling the coolant flow (N₂) on the secondary side. The energy which is transferred between the compressed N₂ and the BOG in the first heat exchanger H10 (pre-heater) will depend on the BOG flow and temperature, and consequently be a more or less fixed value [kW] as long as the BOG flow is constant. This means that the temperature of the N₂ flow exiting the pre-heater H10 will vary with the N₂ flow rate. As long as the heat transfer area of the pre-heater is large enough, the three-way valve V12 (or equivalent flow split constellations) in the N₂ stream upstream the pre-heater H10 can be used for two different purposes:

A: For Thermodynamic Optimization of the Overall Process:

The freedom represented by the flow split (three-way valve V12) can be used to ensure a very efficient heat exchange (low LMTD [log mean temp difference], and consequently low exergy losses) in the upper parts of the cold box H20. The heating and cooling curves can in theory be designed to be parallel with a constant temperature difference between streams at any temperature in the upper (warm) parts of the cold box.

Since the Brayton cycle is based on the concept that pressurized N₂ has a higher heat capacity than low pressure N₂, the heating curves can only be made parallel if the high pressure mass flow is smaller than the cold, low pressure flow. The split of the high pressure stream will consequently cause a very efficient heat exchange in the upper parts of the cold box, and since the branch flow also is cooled independently in the BOG pre-heater, the energy penalty which otherwise would have been associated with the mixing of the two high pressure N2 streams at a lower temperature is reduced to a minimum.

The flow split will typically be controlled based on the BOG compressor suction temperature.

B: For Reducing Thermal Stress in the Cold Box to a Minimum

Another benefit of the flow split control made possible by the three-way valve V12 (or alternative flow split constellations), is that the temperature of the high pressure N₂ stream exiting the pre-heater H10 and flowing in the line 60, can be monitored and, if necessary, controlled in order to avoid rapid temperature fluctuations in the flow which is reintroduced to the cold box via the line 61.

The cold box is normally made in aluminium and is sensitive to thermal stress. By applying a safety control function which changes the flow through the pre-heater based on undesirable conditions, the temperature of all streams entering the cold box can be carefully controlled. This would not have been possible if the pre-heater was a low pressure BOG vs. high pressure BOG heat exchanger, as the high temperature BOG outlet temperature would change synchronously with the fluctuation in the low pressure incoming BOG.

Normally, the split ratio defining the flows of streams 57 and 59, will be adjusted in order to extract as much low temperature duty as possible from the low temperature BOG. However, this configuration also opens for controlling the split ratio with respect to the temperature of the nitrogen stream 61 entering the cold box' middle section. Doing so, conditions which may expose the main heat exchanger H20 to damaging thermal stresses can easily be eliminated.

To achieve the optimal heat integration from a thermodynamic point-of-view, the heat exchangers H10 and H20 can be combined in one single multi-pass heat exchanger. However, since the main heat exchanger (cold box) H20 typically will be a plate-fin heat exchanger, which to some extent is sensitive to both rapid temperature fluctuations and large local temperature approaches, it can be feasible to extract some of the heat transfer to an external heat exchanger of a more robust type, as shown at the pre-heater H10 in FIG. 1.

The heat exchanger configuration shown in FIG. 1 will also dampen the temperature fluctuations of the flow 61 entering the main heat exchanger's H20 middle section, since the N₂-coolant stream will be very large compared to the BOG flow. This will ensure a much safer operation with respect to thermal stresses in the cold box.

2. Ambient Temperature BOG Compressor

The main incentive for employing ambient temperature BOG compression is the possibility this offers for rejecting heat to the ambience. While today's commonly used BOG compressors preserves the compression heat within the BOG stream, the compression heat can now be delivered to an external source operating at ambient or near ambient temperatures (e.g. cooling water).

Ambient temperature compression also offers other benefits. Since an aftercooler H13 as shown in FIG. 1 typically will be associated with this system, the temperature of the compressed stream 8 entering the cold box is stabilized relative to the heat rejection source's temperature. After- and intercooling also represent major advantages with respect to operation in recycle and/or anti surge modes, where the external cooling media ensures stable operation, normally without any additional temperature control.

Ambient temperature BOG compression is especially favourable for LNG vessels where boil-off rates, compositions, temperatures and pressures may vary considerably with the type of voyage (ballast or laden voyages) and cargo. Inter- and aftercooling towards ambient conditions will stabilize the compression conditions and ease capacity control (recycling, etc.)

3. Benefits of Selecting a Higher Pressure Ratio

A “higher” pressure ratio over the BOG compressors C11,C12,C13 will in this context relate to a higher cold box inlet pressure in the line 8 than what is strictly necessary to provide a sufficient differential pressure for forcing the LNG back to the cargo tanks.

This allows the cryogenic separator F10 to be placed at an intermediate pressure level, typically limited to a zone between two valves V10, V11 as shown in FIG. 1. The pressure in this zone can then be controlled independently of the BOG compressor discharge pressure and the cargo tank pressure. Accordingly, some of the overall system's capacity control can be performed by pressure adjustments in this region. It will consequently enable the operator or the automated control system to adjust both the amount of vent gas generated as well as the vent gas composition in order to operate under the most economically favourable conditions during all LNG price fluctuations.

A dedicated line can also be placed in order to bypass the separator under conditions where reliquefied BOG is so much subcooled that the separation pressure otherwise will drop below a defined minimum value.

The pressure differential between the main heat exchanger H20 and the separator F10 ensures that the separator can be placed more independent of the main heat exchanger.

A higher BOG compressor discharge pressure will increase the gain (either in form of a higher adiabatic temperature change or reduced flash gas generation) during the throttling processes down to tank pressure.

Last, a higher process pressure will increase the heat transfer coefficient in heat the main heat exchanger H20 and ensure that the condensation here will be performed at higher temperatures in order to reduce exergy losses.

The person skilled in the art will appreciate that the purpose of the three-way valve V12 is to selectively control the flow split between (i) the line 59 connected to the first heat exchanger H10 and (ii) the line 57 connected to the cold box H20. To this end, the three-way valve V12 described above may be replaced by e.g. a controllable choke valve in the line 60, downstream of the first heat exchanger H10, and a fixed-dimension restriction in the line 57. 

1. A method of pre-heating LNG boil-off gas (BOG) stream flowing from a reservoir in a reliquefaction system, prior to compression, the method comprising heat exchanging the BOG stream in a first heat exchanger, against a second coolant stream having a higher temperature than the BOG stream, the method being characterized in that: the second coolant stream is obtained by selectively splitting a first coolant stream into said second coolant stream and a third coolant stream, said third coolant stream being flowed into a first coolant passage in a reliquefaction system cold box, whereby the BOG has reached near-ambient temperatures prior to compression and heat exchange with low temperature BOG is done by optimizing the split of the coolant in the first heat exchanger in order to minimize energy losses, and thermal stresses in the cold box are reduced.
 2. The method of claim 1, wherein the selective splitting of the first coolant stream is performed upstream of the first heat exchanger.
 3. A method for cooling an LNG boil-off gas stream in a reliquefaction plant, the BOG flowing from a reservoir, the method comprising: compressing the BOG; heat exchanging the compressed BOG against a coolant in a cold box; flowing substantially re-liquefied BOG from the cold box to the reservoir; characterized by prior to the compression step, pre-heating the BOG to substantially ambient temperatures, by heat exchanging the BOG with said coolant, said coolant prior to the heat exchange having a higher temperature than the BOG.
 4. The method of claim 1, wherein the necessary duty to heat the BOG prior to compression is transferred from the coolant stream, downstream of a coolant compander aftercooler but upstream of the cold box.
 5. The method of claim 3, wherein a portion of the coolant stream to the BOG pre-heater, at a point between the coolant compander and the pre-heater, is routed into a dedicated flow path in the cold-box before it is mixed with the coolant stream flowing from the pre-heater.
 6. The method of claim 3, wherein the pressure of the reliquefied BOG between the cold box and the reservoir is controlled independently of the BOG compressor discharge pressure and the reservoir pressure, and the amount of vent gas generated and the vent gas composition thus may be controlled.
 7. An apparatus for cooling an LNG boil-off gas in a reliquefaction system, comprising: a closed-loop coolant circuit for heat exchange between a coolant and the BOG; a BOG compressor having an inlet side fluidly connected to an LNG reservoir; a cold box having a BOG flowpath with a BOG inlet fluidly connected to the BOG compressor outlet side; said BOG flowpath having outlet for substantially re-liquefied BOG, fluidly connected to the reservoir; said cold box further comprising coolant flowpaths for heat exchange between the BOG and the coolant, characterized by a first heat exchanger in the fluid connection between the reservoir and the BOG compressor inlet side, said first heat exchanger having a coolant path fluidly connected to the closed-loop coolant circuit, at a point downstream of the coolant circuit's compander aftercooler but upstream of the coolant flow paths in the cold box, whereby the BOG compressor receives BOG with temperatures near or at the system ambient temperatures.
 8. The apparatus of claim 7, further comprising: a selector valve in the coolant circuit, in a line downstream of the compander aftercooler, and a coolant line at one end connected to a first outlet of the selector valve and at the other end connected to the inlet of the coolant passage of the first beat exchanger, and coolant line, at one end connected to a second outlet of the selector valve and at the other end connected to the inlet of a first coolant passage in the cold box.
 9. The apparatus of claim 7, wherein the first heat exchanger coolant path fluid connection further comprises a coolant line at one end connected to the outlet of the coolant passage of the first heat exchanger and at the other end connected to a line fluidly connected to the outlet of the second heat exchanger first coolant passage, and wherein said lines are connected to the inlet of a second coolant passage in the second heat exchanger.
 10. The apparatus of claim 7, further comprising a separator in fluid connection with the cold box outlet and with the reservoir, a first valve in the cold box outlet line and a second valve in a line connected to the reservoir, said separator also comprising a vent line, whereby the pressure in the separator may be controlled, and the amount of vent gas and the vent gas composition thus may be adjusted. 